Windows 8 has once again come under fire over its questionable user interface. This time, it's Jakob Nielsen from the Nielsen Norman Group voicing an opinion based on a recent case study conducted by the company. Simply put, Nielsen's words aren't very flattering.

The group invited 12 experienced PC users to try out Windows 8 on the new Surface RT tablet as well as a traditional computer. Sure the sample group was small but regardless, he was able to draw some pretty telling conclusions based on the users' feedback.

Starting with the user interface, Nielsen likens Windows 8 to the story of Dr. Jekyll and Mr. Hyde - referring of course to the PC-oriented desktop environment alongside the tablet-minded (Metro) interface.

Specifically, he said Windows 8 on mobile devices and tablets is like a tortured soul hoping for redemption and that on a regular PC, it's like a monster that terrorizes poor office workers and strangles their productivity.

Having two environments on one device can lead to a number of problems, including having to remember where to go for which features. He also points out that switching between two environments will increase the interaction cost of using multiple features and because the two environments work differently, it will make for an inconsistent user experience.

In summary, Nielsen says Windows 8 has several hidden features, reduced discoverability, has cognitive overhead from dual environments and has reduced power from a single-window user interface and low information density. Perhaps these are some of the reasons that Windows 8 sales are below Microsoft's projections at this point.